The present proposal explores a novel conceptualization of glucocorticoid (GC) action in the stress response during a fight/flight emergency. This is that stress-induced GCs function to alert the organism's CNS innate immune system to potential danger signals that include 1) exogenous pathogens and/or pathogen associated molecular patterns (PAMPs), & 2) endogenous danger signals or alarmins, which can be released in response to a wide array of stimuli including sterile injury. Here, GCs prime or sensitize the CNS innate immune/inflammatory response to subsequent stimulation, such as occurs upon exposure to infectious agents or injury. The idea is that stress-induced GCs can function as an endocrine warning signal to the CNS innate immune system to induce a preparatory response (microglial sensitization) to subsequent proinflammatory stimuli. Neuroinflammatory processes are a focus here given the emerging roles of CNS pro-inflammatory cytokines in the etiology of psychiatric disorders in which stress & GCs are key etiologic factors. The primary objectives are: 1) to explore how stress sensitizes microglia to pro-inflammatory challenges & 2) examine whether GCs mediate stress-induced sensitization of microglia. The general hypotheses to be tested are that A) stress sensitizes microglia to pro-inflammatory stimuli by upregulating pattern recognition receptors (PRRs) on microglia. The PRRs Toll-Like receptor (TLR) 2 and/or 4 transduce the pro-inflammatory cytokine effects of endogenous danger signals. We propose that stress primes microglia by upregulating TLR 2 and/or 4 on microglia for a period of days post-stress. Subsequently, upregulated TLRs expressed by microglia sense endogenous danger signals released upon exposure to a pro- inflammatory stimulus. B) TLR mediation requires that the later pro-inflammatory stimulus increase an endogenous molecule in the brain that acts at the TLRs. High Mobility Group Box1 (HMGB1), an alarmin released in response to danger, is an endogenous protein in the CNS that acts at TLRs & we suggest that HMGB1 is increased by a pro-inflammatory stimulus & then acts at the stress-upregulated TLRs, thereby inducing a potentiated pro-inflammatory cytokine response. C) GCs mediate the stress-induced sensitization of microglia (i.e. upregulation of TLR 2/4). Clinically, stress and neuroinflammatory processes are emerging as important factors in the etiology of psychiatric disorders. Also, an extensive body of evidence implicates GCs in the etiology of such disorders. The proposed work may provide valuable insight into how stress/GCs regulate neuroinflammatory processes in psychiatric disorders. Thus, the present research may lead to a reconceptualization of stress/GCs as neuroinflammatory predisposing factors in the etiology of psychiatric disorders (i.e. major depression, PTSD). PUBLIC HEALTH RELEVANCE: Stress is a key factor in the development of mental health disorders and has been found to increase brain inflammation. Likewise, inflammation in the brain is also considered a key factor in the development of mental health disorders such as major depression. The present research studies how stress predisposes people to mental health disorder by focusing on how stress increases brain inflammation.